1. Field of the Invention
The present invention relates to a method for detecting a target compound using magnetic nano-particles, and in particular to a method for selectively detecting bound magnetic nano-particles to which the target compound is bound in a liquid mixture in which magnetic nano-particles to which the target compound is bound and magnetic nano-particles to which the target compound is not bound are mixed together.
2. Description of the Related Art
Means using magnetic fine particles having a size of several μm have been proposed as means for effectively collecting a minute amount of a target compound. Examples of commercially available products include DYNABEADS, which are polystyrene beads (with a particle diameter of several μm) in which ferrite fine particles are dispersed. Such magnetic fine particles are used as precise detection means for detecting biological substances since the magnetic fine particles can be simply and effectively collected using an external magnetic field (for example, see International Publication No. WO 00/05357, Japanese Patent Application Laid-Open (JP-A) No. 5-292971, Japanese Patent Application National Publication (Laid-Open) No. 2003-523185, and Bio Industry, 2004, Vol. 21, No. 8, pp. 39-47). However, although the responsiveness to a magnet is good since the particle diameter is large, the adsorption amount and analytical sensitivity of desired substances (target compounds) are not sufficient. On the other hand, there is another problem in that separation with a magnet becomes difficult when the particle diameter is reduced to several tens of nanometers or less since the ferromagnetism changes to superparamagnetism. Thermal stimulation responsive magnetic nano-particles taking advantage of polymers having a lower critical solution temperature (LCST) or an upper critical solution temperature (UCST) have been proposed as means for effectively separating a target compound using magnetic nano-particles (see, for example, International Publication Nos. WO 02/16571 and WO 02/16528, and JP-A No. 2002-60436).
However, in order to efficiently detect a target compound using the magnetic nano-particles responsive to external stimulation such as heat, a so-called “washing” step is essential, wherein the target compound bound to the magnetic nano-particles is once trapped by a magnetic force, and the target compound bound to the magnetic nano-particles is selectively extracted by washing other samples away.